1. Field of the Invention
The present invention relates to an ultrasonic operation system, or more particularly, an ultrasonic operation system characterized by a control action of locking an output frequency on to the resonance frequency of an ultrasonic transducer so as to drive the ultrasonic transducer at the resonance frequency.
2. Related Art Statement
In general, an ultrasonic transducer employed in an ultrasonic knife for surgical use or in an ultrasonic aspirator should preferably be driven at the resonance frequency of the ultrasonic transducer or a frequency close to the resonance frequency. Among related arts, a known art is implemented in a driving apparatus including, for example, a phase-locked loop (PLL). In the driving apparatus, the phase of an induced current is compared with the phase of a driving voltage applied to the ultrasonic transducer, so that a driving frequency at which the ultrasonic transducer is driven will be agreed with the resonance frequency of the ultrasonic transducer.
This type of ultrasonic transducer is expressed as an equivalent circuit shown in FIG. 24 over a frequency band covering the resonance frequency that serves as a reference frequency. As illustrated, a coil Ld is connected in parallel or series with a braking capacitor Cd included in the ultrasonic transducer. Herein, the inductance of the coil Ld and the capacitance of the capacitor Cd have a relationship of L×C=Ld×Cd to the inductance of another coil L and the capacitance of another capacitor C to as to cancel the capacitance of the capacitor Cd. At the resonance frequency fr=½π√(L×C) (=½π(L×C)½), the property of the ultrasonic-transducer is dominated by only a forward resistance of a resistor R. A phase difference-between an applied voltage and an induced current becomes zero. FIG. 25 graphically shows an impedance Z offered by the ultrasonic transducer over a frequency band centered on the resonance frequency fr.
A PLL is activated in order to raise or lower a driving frequency, at which the ultrasonic transducer is driven, so that the phase difference between the phase θv of an applied voltage and the phase θi of an induced current will become zero. Consequently, a driving apparatus including the PLL is locked on to the resonance frequency fr. FIG. 26 shows the phase difference.
Herein, a frequency at which the driving apparatus is activated must fall within a range from a frequency f1 to a frequency f2 within which the driving apparatus can be locked on to the resonance frequency. Japanese Unexamined Patent Publication No. 2691011 has disclosed an improvement in which the frequency at which the driving apparatus is activated falls within the range.
FIG. 27 is an explanatory diagram concerning the disclosed related art. An ultrasonic operation system consists mainly of an ultrasonic transducer 1101, and a driving apparatus composed of a PLL 1102, an amplification circuit (AMP) 1103, a detection circuit 1104, an oscillatory circuit 1105, and a switch 1106. The ultrasonic transducer 1101 exhibits a resonance frequency of fr. The PLL 1102 tracks the resonance frequency according to the phase of a driving voltage applied to the ultrasonic transducer 1101 and the phase of an induced current. The amplification circuit 1103 amplifies the power of a frequency-change signal output from the PLL 1102 so as to produce driving power with which the ultrasonic transducer 1101 is driven. The detection circuit 1104 detects the applied voltage and induced current. The oscillatory circuit 1105 produces a reference-frequency signal that represents a reference frequency at which,a driving apparatus is activated. The switch 1106 switches the connections of one input terminal of the PLL 1102 depending on whether the switch receives the induced current or reference-frequency signal. When the driving apparatus is activated, the reference-frequency signal (fref) output from the oscillatory circuit 1105 is input to the PLL 1102. Therefore, a voltage locked on to the reference frequency is applied to the ultrasonic transducer 1101. Thereafter, the connections of the one terminal of the PLL 1102 are switched in order to select the induced current as an input of the PLL 1102. Thus, the driving apparatus is locked on to the resonance frequency.
The reference-frequency signal produced by the oscillatory circuit 1105 is adjusted to fall within the frequencies f1 and f2. When the connections through the switch 1106 are changed, a driving frequency at which the ultrasonic transducer is driven falls within a frequency band within which the driving apparatus can be locked on to the resonance frequency. Therefore, the driving apparatus can reliably perform the action of locking the driving frequency on to the resonance frequency.
In an operation system including an ultrasonic knife, probes having various shapes are connected to an ultrasonic transducer and adopted for different purposes of use. Moreover, a plurality of types of ultrasonic transducers that exhibit different resonance frequencies may be used with employment of only one driving apparatus. In this case, handpieces each composed of a transducer and a probe exhibit different resonance frequencies. Consequently, a range from a frequency f1 to a frequency f2 within which a driving apparatus can be locked on to the resonance frequency of an ultrasonic transducer is different from handpiece to handpiece. A frequency at which the driving apparatus is activated is swept over a range covering the resonance frequency. Meanwhile, it is detected whether the frequency becomes equal to the resonance frequency. As soon as the frequency becomes equal to the resonance frequency, a PLL is activated to perform a phase locking action. This art is described in Japanese Patent Publication No. 2647713.
However, in the foregoing related art, a circuit for producing a frequency-change signal that represents a frequency at which the driving apparatus is activated and a PLL are of an analog type. It is therefore very hard to suppress an adverse effect caused by a difference of one product from another or to adjust each product. Moreover, when an additional probe or ultrasonic transducer is included, if a frequency at which the probe or ultrasonic transducer should be driven is unprecedented, a frequency setting circuit incorporated in the driving apparatus must be readjusted. This poses a problem.
Moreover, several digital resonance-frequency tracking circuits that are controlled by a CPU have been proposed in efforts to compensate for the drawbacks of the analog type. Since a tracking action depends on computation performed by software, the digital type has a drawback that a control action is performed slowly. If a fast CPU is employed in order to overcome the drawback, a driving apparatus itself becomes expensive. This poses a problem.
Furthermore, when the driving apparatus is applied to an ultrasonic operation system, it is important whether the driving apparatus can respond quickly when activated for oscillation. The driving apparatus must be designed to offer a high response speed.